In the case of a power semiconductor module such as this which is disclosed in DE 199 42 915 AI, two or more power semiconductors are arranged in a row on the upper face of an isolating and thermally conductive mount (substrate), and are connected to conductor tracks which run on the upper face of the substrate.
The lower face of the substrate is pressed against a heat sink by a pressing apparatus.
Power losses which occur in the form of heat during operation of the power semiconductor module are dissipated via the heat sink. For effective heat dissipation and a low thermal contact resistance, and hence reliable operation of the power semiconductor module, the heat sink must rest flat on the substrate lower face, without any gaps.
One problem in this case is the internal mechanical stresses on the module resulting from the different thermal coefficients of expansion of the different materials in the semiconductor module components (for example of the substrate and semiconductor material).
These stresses lead to undesirable deformation of the substrate and power semiconductor module lower face, so that a flat contact surface is no longer guaranteed. This results in intermediate spaces and air gaps, which adversely affect the heat transmission between the heat sink and the substrate. This problem becomes worse as the substrate size increases.
In order to solve this problem, it is conceivable to additionally provide a metal plate as a base plate, to whose upper face the substrate lower face, for example, is soldered. The intermediate solder layer would then compensate for shape discrepancies. The lower face of the base plate would be connected to the heat sink in order to provide a uniform heat distribution (as a so-called heat spreader) and to absorb mechanical stresses. However, this design increases the total costs of a power semiconductor module designed in this way, as a result of the additional base plate and its fitting.
It is also feasible to increase the contact forces by means of external brackets, such as those which are known in principle, for example, from DE 197 23 270 AI. However, if the substrate is severely loaded by high local contact pressures, there is a risk of the substrate fracturing. This risk increases as the substrate size increases. Furthermore, the use of additional brackets complicates the assembly process, and makes it more expensive.